1. Field of the Disclosure
The present disclosure relates to a separator for lithium battery, and more particularly to a separator with a middle layer formed by a punch method and a manufacturing method thereof.
2. Description of the Related Art
In general, a battery used for power supply comprises a positive electrode, a negative electrode, an electrolyte and a separator. The separator is disposed between the positive electrode and the negative electrode to electrically space the positive electrode and the negative electrode apart from each other, so as to prevent electrical short circuit or even an unexpected explosion in the battery. In addition, a separator includes a plurality of micropores used to accommodate the electrolyte.
In case a battery separator fails to electrically insulate the positive electrode from the negative electrode, the battery is potentially subject to a short circuit which is likely to turn into a thermal runaway and even an explosion. A microporous polyolefin separator with multi-layer structure has been developed for use in the battery (e.g., a secondary lithium battery). While the temperature of the battery is high enough to the melting point of anyone of layers of the microporous polyolefin separator, the layer is melted to shutdown the micro pores of the microporous polyolefin separator. As a result, the lithium ion flow of the battery is blocked, which interrupts the current of the battery. It is needed to decrease the risk of the battery subject to thermal runaway or even the explosion.
Currently, the separators in the market are made of polyethylene and/or polypropylene in a single layer, multi-layers or a composite layer structure. For example, U.S. Pat. No. 5,691,077 discloses a tri-layer battery separator constructed by polypropylene-polyethylene-polypropylene layers and a manufacture method for the same. The manufacture method comprises steps of: forming a polypropylene precursor; forming a polyethylene precursor; stretching the polypropylene and polyethylene precursor to form a polypropylene microporous film and a polyethylene microporous film; bonding the two polypropylene microporous film and the polyethylene microporous film in a order of polypropylene, polyethylene, and polypropylene films, bonding or hot pressing to generate a three-layer battery separator. The three-layer battery separator has a shutdown temperature about 130° C.
The melting point of polypropylene is higher than that of polyethylene, and the breakdown strength of the polypropylene is stronger than that of the polyethylene. In this regard, if the internal temperature of the battery with tri-layer separator (polypropylene/polyethylene/polypropylene) reaches a temperature of 130° C., the microporous polyethylene of the tri-layer separator will be melted and be blocked, while the microporous polypropylene film maintains the structure and mechanical property thereof. It still keeps the positive electrode and the negative electrode electrically insulated apart from each other and thus may maintain the safety of the battery. However, in case the internal temperature of the battery is raised and kept in about 130° C. for a period of time, even though the outer polypropylene microporous film maintains the structure and mechanical property of the separator, the separator of microporous polypropylene and polyethylene films may start to shrink. The edge regions of the two electrodes may be exposed to each other and increase the risk of the thermal runaway the explosion of the battery.
Therefore, the present disclosure provides a method for manufacturing separators with a middle layer formed by a punch method. Because no stretch processing is applied in the formation of the microporous structure of the middle layer, the separator with a punched middle layer has a better heat resistant and a high mechanical strength at the elevated temperature.